Thin glass sheet with tunable coefficient of thermal expansion

ABSTRACT

A glass sheet is formed using a roll-to-roll glass soot deposition and sintering process. The glass sheet formation involves forming a first glass soot layer on a deposition surface of a soot-receiving device, removing the first glass soot layer from the deposition surface, and forming a second glass soot layer on the unsupported first glass soot layer. The resulting composite glass soot sheet is heated to form a sintered glass sheet. The glass sheet can be a substantially homogeneous glass sheet or a composite glass sheet having layer-specific attributes.

BACKGROUND

The present disclosure relates generally to glass sheets and more specifically to a glass soot deposition and sintering process for forming uniform glass sheets having a desired composition and coefficient of thermal expansion.

Glass sheet materials can be formed using a variety of different methods. In a float glass process, for example, a sheet of solid glass is made by floating molten glass on a bed of molten metal. This process can be used to form glass sheets having uniform thickness and very flat surfaces. However, float glass processes necessarily involve direct contact between the glass melt and the molten metal, which can lead to undesired contamination at the interface and less than pristine surface quality. In order to produce high quality float glass sheets with pristine surface properties on both major surfaces, float glass is typically subjected to one or more surface polishing steps. This processing adds additional expense. Moreover, it is believed that the float process has not been used to make rollable (i.e., very thin) glass sheets.

An additional method for forming glass sheet materials is the fusion draw process. In this process, molten glass is fed into a trough called an “isopipe,” which is overfilled until the molten glass flows evenly over both sides. The molten glass then rejoins, or fuses, at the bottom of the trough where it is drawn to form a continuous sheet of flat glass. Because both major surfaces of the glass sheet do not directly contact any support material during the forming process, high surface quality in both major surfaces can be achieved.

Due to the dynamic nature of the fusion draw process, the number of glass compositions suitable for fusion draw is limited to those that possess the requisite properties in the molten phase (e.g., liquidus viscosity, strain point, etc.). Finally, the apparatus used in the fusion draw process can be expensive.

Thin glass sheets having a coefficient of thermal expansion (CTE) closely matched to silicon can be formed by conventional float and fusion draw processes. It is impractical, however, to modify the glass composition in these processes in order to achieve a particular coefficient of thermal expansion in the resulting glass, even assuming a particular glass composition was determined to be fusion drawable. Notably, it would be time consuming and expensive to reconfigure the fusion draw process apparatus, including the melt tanks, fusion isopipes and glass conveyance equipment for each new glass composition. Moreover, the potential for cross-contamination issues make it difficult to even consider routinely changing the glass composition in a fusion draw process in order to produce thin glass sheets having a desired coefficient of thermal expansion.

SUMMARY

In view of the foregoing, economical, uniform, thin glass sheets having a high surface quality and a tailored coefficient of thermal expansion are highly desirable. The process disclosed herein can produce glass sheets to a specified thickness and composition without the need for post-formation lapping or polishing. The glass sheets can comprise one or more layers, components, or phases. Such glass sheets can be used, for example, as substrates for microelectronics, and as substrates for forming superconducting materials, etc.

A method of forming glass sheets involves a glass soot deposition and sintering process. According to various embodiments, the process involves depositing first glass soot particles on a deposition surface of a soot-receiving device to form a supported soot layer, removing the soot layer from the deposition surface to form a first soot sheet, and depositing second glass soot particles on at least one surface of the first soot sheet to form a composite soot sheet. The second glass soot particles can be sintered to form a glass sheet in a sintering step that does not sinter the first glass soot particles. The resulting glass sheet, which can be separated from the first soot sheet, can be a thin, rollable glass sheet, or a thicker glass sheet.

As will be appreciated, the supported soot layer that is released from the deposition surface has two major opposing surfaces. The “contact surface” is the surface that is formed in contact with the deposition surface of the soot receiving device, while the “free surface” is the opposing surface. In a further embodiment, after the supported soot layer is released from the deposition surface to form a first soot sheet, second glass soot particles can be deposited on one or both of the “contact surface” or the “free surface.” In the context of the thus formed composite soot sheet, an interfacial surface means a surface between one soot layer and a subsequently-deposited soot layer, while a free surface means a surface that has been formed without direct contact with another soot layer.

In embodiments, a difference in the sintering temperature between the first glass soot particles and the second glass soot particles enables the second glass soot particles to sinter while in contact with one of the major opposing surfaces of the first soot sheet. As a result of a higher sintering temperature of the first glass soot particles, only the second glass soot particles sinter to form a glass sheet. The glass sheet made using the foregoing approach can have an average thickness of 200 microns or less and an average surface roughness over at least one of two major opposing surfaces of 1 nm or less. By controlling the composition of the second glass soot particles, the CTE of the resulting glass sheet can also be controlled.

Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus for forming a thin glass sheet; and

FIG. 2 is a schematic illustration of a composite soot sheet.

DETAILED DESCRIPTION

An apparatus for forming a glass sheet according to one embodiment is shown schematically in FIG. 1. The apparatus 100 comprises a pair of soot-providing devices 110, 116, a soot-receiving device 120, a soot sheet-guiding device 130, and a soot sheet-sintering device 140.

As will be disclosed in further detail below, embodiments of the disclosure relate to the formation of a thin, high CTE glass sheet from a second soot sheet that is formed on a first soot sheet. During a heating step in which the second soot sheet is sintered, the first soot sheet is largely unaffected by the heating. Thus, high CTE glasses, which typically have a relatively low melting (and sintering) temperature, can be formed using the disclosed soot-based process wherein a previously-formed soot layer (first soot layer) is used as a disposable carrier for the formation and densification of the second soot sheet. Without the first soot sheet as a carrier, the second glass soot particles that form the second soot sheet would sinter and undesirably bond to the soot-receiving device rather than form a free-standing soot sheet. Thus, embodiments of the disclosure relate to the successive formation of two soot sheets. The first soot sheet, which forms the carrier sheet, comprises a high melting temperature, high sintering temperature material, and the second soot sheet comprises a relatively low melting temperature, low sintering temperature material.

In an initial deposition step, first glass soot particles 150 formed by a first soot-providing device 110 are deposited on a deposition surface 122 of the soot-receiving device 120. The soot-receiving device 120 may be in the form of a rotatable drum or belt and thus can comprise a continuous deposition surface 122. The soot particles 150 are deposited to form a soot layer 152 on the deposition surface 122. The soot layer 152 can be released from the deposition surface 122 as a free-standing, continuous soot sheet 154 having a contact surface 155 and a free surface 157. The act of releasing the soot layer 152 from the deposition surface 122 can occur without physical intervention as the result of, for example, thermal mismatch or a mismatch in coefficients of thermal expansion between the soot layer and the deposition surface and/or under the effect of the force of gravity.

After the first soot sheet 154 is released from the soot-receiving device 120, a soot sheet-guiding device 130 can guide movement of the first soot sheet 154. In an embodiment, in a second deposition step, second glass soot particles 160 formed by a second soot-providing device 116 are deposited on the contact surface 155 of first soot sheet 154 to form a second soot layer 162. The addition of the second soot layer 162 and any subsequent soot layer (not shown) to the first soot sheet 154 forms a composite soot sheet 174. The soot sheet-guiding device 130 can further guide movement of the composite soot sheet 174 through a soot sheet-sintering device 140, which sinters and consolidates a portion of the composite soot sheet 174 to form a glass sheet 156. In embodiments, a second soot layer can be deposited on the free surface 157 of the soot sheet 154 in addition to or in lieu of forming a second soot layer on the contact surface 155 of the soot sheet 154.

Thus, a process of forming a glass sheet comprises depositing glass soot particles on a deposition surface of a soot-receiving device to form a supported soot layer, removing the soot layer from the deposition surface to form a first soot sheet, and depositing additional glass soot particles on at least one surface of the first soot sheet to form a composite soot sheet. The composite soot sheet can be heated to form a sintered glass sheet from the second glass soot particles 160. Additional aspects of the process and apparatus are disclosed in detail herein below.

Although a variety of devices may be used to form glass soot particles, by way of example, soot providing devices 110, 116 may comprise one or more flame hydrolysis burners, such as those used in outside vapor deposition OVD, vapor axial deposition (VAD) and planar deposition processes. Suitable burner configurations are disclosed in U.S. Pat. Nos. 6,606,883, 5,922,100, 6,837,076, 6,743,011 and 6,736,633, the contents of which are incorporated herein by reference in their entirety.

The soot-providing devices 110, 116 may comprise a single burner or multiple burners. An example burner has an output surface having length l and width w. The output surface comprises N columns of gas orifices where N can range from 1 to 20 or more. In an embodiment, each orifice comprises a 0.076 cm diameter hole. The length l of the output surface can range from about 2.5 to 30.5 cm or more, and the width can range from 0.1 to 7.5 cm. Optionally, multiple burners can be configured into a burner array that can produce a substantially continuous stream of soot particles over the length and width of the array.

A burner array, for example, may comprise a plurality of individual burners (e.g., placed end-to-end) configured to form and deposit a temporally and spatially uniform layer of glass soot. Thus, each soot-providing device can be used to form an individual layer of soot having a substantially homogeneous chemical composition and a substantially uniform thickness. By “uniform composition” and “uniform thickness” is meant that the composition and thickness variation over a given area is less than or equal to 20% of an average composition or thickness. In certain embodiments, one or both of the compositional and thickness variation of a soot sheet can be less than or equal to 10% of their respective average values over the soot sheet.

An example burner comprises 9 columns of gas orifices. In forming silica glass soot, for example, according to one embodiment, the centerline column (e.g., column 5) provides a silica gas precursor/carrier gas mixture. The immediately adjacent columns (e.g., columns 4 and 6) provide oxygen gas for stoichiometry control of the silica gas precursor. The next two columns of gas orifices on either side of the centerline (e.g., columns 2, 3, 7 and 8) provide additional oxygen, the flow rate of which can be used to control stoichiometry and soot density, and provide an oxidizer for the ignition flame. The outermost columns of orifices (e.g., columns 1 and 9) can provide an ignition flame mixture of, for example, CH₄/O₂ or H₂/O₂. Example gas flow rate ranges for such a 9 column linear burner are disclosed in Table 1. A silicon-containing gaseous precursor can be used to form a first soot sheet, upon which a second soot sheet can be formed.

TABLE 1 Example gas flow rates for 9 column linear burner to form silica soot sheet Burner Gas column(s) Example flow rate OMCTS 5 15 g/min N₂ 5 40 SLPM O₂ 4, 6 18 SLPM O₂ 2, 3, 7, 8 36 SLPM CH₄ 1, 9 36 SLPM O₂ 1, 9 30 SLPM

The soot-providing devices may be held stationery during formation and deposition of the soot particles or, alternatively, the soot-providing devices may be moved (e.g., oscillated) with respect to the deposition surface. A distance from the burner output surface to the deposition surface can range from about 20 mm to 100 mm (e.g., 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 mm).

Operation of the soot-providing device typically involves chemical reactions between precursor chemicals (e.g., gaseous compounds or atomized liquids vaporized in the flame) to form glass soot particles. Optionally, the chemical reactions can be further assisted by supplemental energy sources such as plasma or a supplemental heating device.

Silicon-containing precursor compounds, for example, can be used to form soot sheets comprising silica soot particles. An example silicon-containing precursor compound is octamethylcyclotetrasiloxane (OMCTS). OMCTS can be introduced into a burner or burner array together with H₂, O₂, CH₄ or other fuels where it is oxidized and hydrolyzed to produce silica soot particles.

As-produced or as-deposited, the soot particles may consist essentially of a single phase (e.g., a single oxide), which can be sintered to form, for example, un-doped, high-purity glass. Alternatively, the soot particles may comprise two or more components or two or more phases, which can be sintered to form, for example, doped glass. Multiphase glass sheets, for example, can be made by incorporating a titanium oxide precursor or a phosphorous oxide precursor into the OMCTS gas flow. Example titanium and phosphorous oxide precursors include various soluble metal salts and metal alkoxides such as halides of phosphorous and titanium (IV) isopropoxide.

In the example of a flame hydrolysis burner, doping can take place in situ during the flame hydrolysis process by introducing dopant precursors into the flame. In a further example, such as in the case of a plasma-heated soot sprayer, soot particles sprayed from the sprayer can be pre-doped or, alternatively, the sprayed soot particles can be subjected to a dopant-containing plasma atmosphere such that the soot particles are doped in the plasma. In a still further example, dopants can be incorporated into a soot sheet prior to or during sintering of the soot sheet. Example dopants include elements from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB and the rare earth series of the Periodic Table of Elements.

The soot particles can have an essentially homogeneous composition, size and/or shape. Alternatively, one or more of the composition, size and shape of the soot particles can vary. For example, soot particles of a main glass component can be provided by one soot-providing device, while soot particles of a dopant composition can be provided by a different soot-providing device. In certain embodiments, soot particles can mix and/or adhere to one another during the acts of forming and depositing the soot particles to form composite particles. It is also possible that the soot particles are substantially prevented from adhering to each other to form mixed particles prior to or while being deposited on a deposition surface.

Referring still to FIG. 1, deposition surface 122 comprises a peripheral portion of the soot-receiving device 120 and can be formed of a refractory material. In an embodiment, the deposition surface 122 is formed of a material that is chemically and thermally compatible with both the first soot particles 150 and the deposited soot layer 152, and from which the soot layer can be easily removed. Example soot-receiving devices 120 comprise a coating or cladding of a refractory material (e.g., silica, silicon carbide, graphite, zirconia, etc.) formed over a core material of, for example, steel, aluminum or metal alloy. Further soot-receiving devices can comprise a unitary part consisting essentially of a suitable refractory material such as quartz.

The soot-receiving device 120 and particularly the deposition surface 122 can be configured in a variety of different ways and have a variety of shapes and/or dimensions. For example, a width of the deposition surface can range from about 2 cm to 2 m, although smaller and larger dimensions are possible. A cross-sectional shape of the soot-receiving device 120 can be circular, oval, elliptical, triangular, square, hexagonal, etc., and a corresponding cross-sectional dimension (e.g., diameter or length) of the soot-receiving device 120 can also vary. For example, a diameter of a soot-receiving device having a circular cross section can range from about 2 cm to 50 cm. An example soot-receiving device 120 comprises a quartz cylinder having a 250 mm inner diameter, a 260 mm outer diameter, and a 24 cm wide deposition surface.

In the examples of circular or oval cross-sections, the deposition surface 122 can comprise a closed, continuous surface, while in the examples of elliptical, triangular, square or hexagonal cross-sections, the deposition surface can comprise a segmented surface. By appropriately selecting the size and dimensions of the soot-receiving device 120, a continuous or semi-continuous soot sheet can be formed.

The deposition surface 122 can be a smooth surface or optionally include regular or irregular patterning in the form of raised or lowered protrusions across a range of length scales. A smooth surface can be used without additional polishing or after one or more post-formation polishing steps. The patterning can range from one or more discrete facets to a general roughing of the surface. A deposited soot layer can conform to the patterning in the deposition surface. A pattern formed in the first soot sheet can be retained in the first soot sheet as it is separated from the deposition surface and, in turn, transferred to and preserved in the sintered surface of the second soot sheet resulting in an embossed glass sheet.

In a variation of the above-described deposition surface-derived embossing, one or both of the contact surface and the free surface of a first soot sheet can be patterned after it is removed from the deposition surface but prior to sintering. Further, a surface of a second soot sheet can be patterned after it is formed over the first soot sheet but prior to sintering. For example, by gently touching a soot sheet surface, Applicants have patterned the soot sheet surface with a fingerprint. Upon sintering of the soot sheet, the fingerprint pattern is retained in the resulting glass sheet.

In certain embodiments, the soot-receiving device 120 is rotated during the act of depositing soot particles 150 in order to form a soot layer 152 thereon. The rotation can be unidirectional, e.g., clockwise or counter-clockwise. A direction of rotation according to one embodiment is indicated by arrow A in FIG. 1. Optionally, the soot-receiving device may oscillate during the soot deposition process, i.e., the rotation direction may change intermittently. A linear velocity of the deposition surface 122 of the soot-receiving device 120 can range from 0.1 mm/sec to 10 mm/sec (e.g., 0.1, 0.2, 0.5, 1, 2, 3, 4, 5 or 10 mm/sec). In scale-up, it is believed that the linear velocity of the deposition surface can be increased up to 1 m/sec or higher.

Soot particles 150 are deposited on only a portion of the deposition surface 122, and the deposited soot layer 152 is removed to form a free-standing continuous or semi-continuous first soot sheet 154 having length L. A width of the deposited layer 152 (and nominally of the soot sheet 154) is W.

In certain embodiments, the soot layer can be continuously formed on and continuously removed from the deposition surface. During formation of a soot layer, soot particles bond to a certain degree with each other and with the deposition surface. The higher the average temperature of the soot particles when they are being deposited, the more likely they are to bond with each other and form a dense and mechanically robust soot sheet. However, higher deposition temperatures also promote bonding between the soot particles and the deposition surface, which can interfere with releasing of the soot sheet.

Bonding between soot particle and the deposition surface can be controlled by controlling a temperature gradient between a location where the soot particles are deposited and a location where the soot layer 152 is released to form the first soot sheet 154. For instance, if the soot layer and the deposition surface have sufficiently different coefficients of thermal expansion (CTEs), the release may occur spontaneously due to stress caused by the temperature gradient. In certain embodiments, removal of the deposited soot layer from the deposition surface can be made easier by forming a soot layer having a width W that is less than the width of the deposition surface 122. To obtain a substantially uniform temperature across the deposition surface, the soot-receiving device can be heated or cooled either from the inside, the outside, or both.

During the act of separating the soot layer from the deposition surface, a direction of motion of the separated soot sheet can be substantially tangential to a release point on the deposition surface. By “substantially tangential” is meant that the direction of motion of the soot sheet relative to a release point on the deposition surface deviates by less than about 10 degrees (e.g., less than 10, 5, 2 or 1 degrees) from a direction that is tangential to the deposition surface at the release point. Maintaining a substantially tangential release angle can reduce the stress exerted on the soot sheet at the release point.

For a soot-receiving device having a circular or oval cross section, the curvature of the deposition surface is a function of the cross-sectional diameter(s) of the soot-receiving device. As the diameter increases, the radius of curvature increases, and stresses in the deposited soot decrease as the shape of the deposited soot sheet approaches that of a flat, planar sheet.

In embodiments, the first soot sheet has sufficient mechanical integrity to support its own mass (i.e., during the acts of removal from the deposition surface, handling, additional glass soot deposition and sintering) without fracturing or tearing. Process variables that can affect the physical and mechanical properties of the first soot sheet include, inter alia, the thickness and density of the soot sheet, the curvature of the deposition surface, and the temperature of the soot layer and first soot sheet during formation.

The first soot sheet 154 comprises two major surfaces, only one of which contacts the deposition surface during formation of the soot layer. Thus, the two major surfaces of the first soot sheet 154 may be characterized and distinguished as the “contact surface” and the opposing “free surface.”

As shown in FIG. 1, first soot sheet 154 has a contact surface 155 and a free surface 157. According to the illustrated embodiment, a second soot providing device 116 forms a stream of second glass soot particles 160 that are deposited on the contact surface 155. Together, as illustrated in FIG. 2, first soot sheet 154 and second soot layer 162, which comprises second glass soot particles 160, form a composite soot sheet 174. As also shown in FIG. 2, as a result of heating the composite soot sheet, the second glass soot particles 160 can sinter to form glass sheet 156, while the first glass soot particles are not sintered and the first soot sheet 154 is substantially unchanged by the heating.

According to un-illustrated embodiments, after first soot sheet 154 is released from the deposition surface, multiple additional soot layers can be deposited thereon. The additional soot layers can be deposited simultaneously or successively on one or both sides of the first soot sheet 154 (i.e., directly in contact with the contact surface 155 or the free surface 157, or directly in contact with one or more previously-deposited soot layers formed over the contact surface or the free surface).

In an embodiment, a composition of second glass soot particles that are deposited on the contact surface of the first soot sheet is substantially identical to a composition of second glass soot particles that are deposited on free surface of the first soot sheet. In an alternate embodiment, a composition of second glass soot particles that are deposited on the contact surface of the first soot sheet is different than a composition of second glass soot particles that are deposited on free surface of the first soot sheet. By forming second soot layers on both sides of first soot sheet 154, it is possible during a single heating step to simultaneously form two glass sheets having the same or different compositions.

According to an embodiment, a composite soot sheet comprises at least two glass soot layers and has a pair of exposed major opposing surfaces. By depositing at least one of the glass soot layers on the contact layer of the original soot sheet, both of the exposed surfaces of the composite soot sheet are free surfaces.

In an example of a first soot sheet comprising at least 90 mole % silica, an average soot density of the soot sheet can range from about 0.3 to 1.5 g/cm³, e.g., from about 0.4 to 0.7 g/cm³, or from about 0.8 to 1.25 g/cm³, and an average thickness of the first soot sheet 154 can range from 10 to 600 μm, e.g., 20 to 200 μm, 50 to 100 μm or 300 to 500 μm.

Subsequently-deposited soot layers (i.e., a soot layer 162 formed from glass soot particles deposited on at least one of the contact surface and the free surface) can each have an average thickness of 10 to 600 μm, e.g., 20 to 200 μm, 50 to 100 μm or 300 to 500 μm. A total thickness of a resulting composite soot sheet can range from 100 μm to 5 cm.

A composition of the second soot sheet can comprise up to 50 wt. % silica, and can include, for example, about 20 wt. % germania, about 20 wt. % P₂O₅, and 10 wt. % alumina.

In certain embodiments, particularly those involving continuous soot sheet and/or sintered glass sheet production, continuous movement of the first soot sheet 154 away from the deposition surface after its release can be aided by a soot sheet guiding device 130. The soot sheet guiding device 130 can directly contact at least a portion of the soot sheet 154 in order to aid movement and provide mechanical support for the soot sheet.

To maintain a high surface quality of the first soot sheet 154, the soot sheet guiding device 130 may contact only a portion (e.g., an edge portion) of the first soot sheet 154. In certain embodiments, the soot sheet guiding device comprises a pair of clamping rollers that can grip an edge portion of the soot sheet and guide the soot sheet through a soot sheet sintering device.

In a similar vein, continuous movement of the composite soot sheet 174 away from the deposition surface can be facilitated by the soot sheet guiding device 130. The soot sheet guiding device 130 can directly contact at least a portion of the composite soot sheet 174 in order to aid movement and provide mechanical support. As shown in FIG. 1, to maintain a high surface quality of the composite soot sheet 174, the soot sheet guiding device 130 may contact only a portion (e.g., an edge portion) thereof.

Using a soot sheet guiding device, a continuous composite soot sheet 174 can be fed into a sintering/annealing zone of a soot sheet sintering device 140 where at least a portion of the composite soot sheet is heated at a temperature and for a period of time sufficient to convert it into partially or fully densified glass. For example, a composite soot sheet comprising a high purity silica first soot sheet and a second soot sheet can be heated at a temperature of less than 1200° C., e.g., from about 700° C. to 1000° C. such that the second soot sheet is sintered to form a dense glass sheet, while the first (silica) soot sheet is unsintered. The sintering temperature and the sintering time can be controlled in order to control the formation of voids and/or gas bubbles within the sintered glass sheet. As an example, the sintering temperature and the sintering time can be controlled in order to form a sintered glass sheet that is essentially free of voids and gas bubbles.

As used herein, sintering refers to a process whereby second glass soot particles are heated below their melting point (solid state sintering) until they adhere to each other. Annealing is a process of cooling glass to relieve internal stresses after it was formed. Sintering and annealing can be carried out sequentially using the same or different apparatus.

In embodiments, a density of the first soot sheet remains substantially unchanged as a result of the sintering step, which causes densification of the second soot sheet. After the second soot sheet is sintered to form a dense glass sheet, the sintered glass sheet can separated from the first soot layer by virtue of, for example, a CTE or thermal mismatch between the respective layers. The unsintered first soot sheet can be discarded or, optionally, sintered at an elevated temperature (e.g., from about 1200° C. to 1700° C.) to form a separate glass sheet.

The glass sheet formation process may be controlled in order to minimize strain (e.g., sagging) of both the soot sheet and the resulting glass sheet. One way to minimize strain is to orient the composite soot sheet substantially vertically during sintering. According to embodiments, an angle of orientation of the soot sheet with respect to a vertical orientation can be less than 15 degrees (e.g., less than 10 or 5 degrees).

Sintering involves passing a composite glass soot sheet through a sintering zone of a soot sheet-sintering device. During sintering, a sintering front, which represents a boundary between sintered and unsintered material, advances across the second soot sheet until it reaches an edge of the soot sheet or another sintering front. According to an embodiment, the second glass soot sheet is heated and sintered across up to 90% of its width (e.g., 20, 50 or 75% of its width).

A suitable soot sheet-sintering device for conducting the sintering can include one of a variety of designs such as, for example, gas oxy-torches, inductive heaters, resistive heaters and lasers. One feature of the designs may be the ability to sinter only a portion of the second glass soot sheet width at any given time such that the segment being sintered can be in contact with and thus be supported by (e.g., tensioned by) either previously-sintered soot or unsintered glass soot sheet along a width direction.

A variety of different soot sheet-sintering devices such as resistive heating and induction heating devices can be used to sinter the soot sheet. The thermal history of both the soot sheet and the glass sheet can affect the final thickness, composition, compositional homogeneity and other chemical and physical properties of the final product. A glass sheet can be formed by applying heat to one or both of the major surfaces of the second soot sheet. During sintering, various parameters can be controlled including temperature and temperature profile, time, and atmosphere.

Though a sintering temperature can be selected by skilled artisan based on, for example, the composition of the second soot sheet to be sintered, a sintering temperature can range from about 700° C. to 1200° C. Further, a homogeneous temperature profile, which is achievable with various heating sources including both resistive and induction heating sources, can be used to create homogeneity within the final glass sheet. By “homogeneous temperature profile” is meant a sintering temperature that varies by less than 20% (e.g., less than 10 or 5%) over a predetermined sample area or sample volume.

In embodiments where an edge portion of the composite soot sheet 174 is held and guided by the soot sheet-guiding device, that edge portion is typically not sintered by the sintering device. For example, in one embodiment, the center 10 cm of a soot sheet having an average thickness of about 400 microns and a total width of 24 cm was heated to produce a sintered glass sheet having a width of about 10 cm and an average thickness of about 100 microns. Prior to sintering, an average density of the soot sheet is about 0.5 g/cm³.

In addition to controlling the temperature and the temperature profile during sintering, the gas ambient surrounding the soot sheet/glass sheet can also be controlled. Specifically, both the total pressure as well as the partial pressure of suitable sintering gases can be selected in order to control the sintering process. In certain embodiments, a controlled gas mixture can comprise one or more active or inert gases such as, for example, He, O₂, CO₂, N₂, Cl₂, Ar or mixtures thereof.

During the act of sintering, the composite soot sheet may be held stationery within a sintering zone or moved continuously or semi-continuously through such a zone. For example, in a continuous glass sheet forming process, a rate of production of the soot sheet as it is released from the soot deposition surface may be substantially equal to a rate of translation of the composite soot sheet through the sintering zone. Sintering may be performed via one or more passes through a sintering zone using the same or different sintering conditions. A linear velocity of the soot sheet through the sintering zone can range from 0.1 mm/sec to 10 mm/sec (e.g., 0.1, 0.2, 0.5, 1, 2, 3, 4, 5 or 10 mm/sec). In scale-up, it is believed that the linear velocity of the deposition surface can be increased up to 1 m/sec or higher. A distance from the heater to the soot surface can range from about 1 mm to 200 mm (e.g., 1, 5, 10, 20, 50, 100 or 200 mm or more).

The disclosed process in various embodiments may be used to form thin glass sheets having high coefficient of thermal expansion. Such materials, because of their correspondingly low melting temperature and low sintering temperature may be difficult to form as a first soot sheet that is released directly from a deposition surface. Rather, according to the present approach, the second glass soot particles, which will form the glass sheet, are formed over a previously-formed soot sheet that comprises a higher melting temperature and thus a higher sintering temperature material that does not sinter during the step of sintering. For example, the second glass soot particles can be formed over a soot sheet comprising fused or glassy silica, which has a coefficient of thermal expansion of about 5×10⁻⁷/° C. According to embodiments, the resulting glass sheets may have a coefficient of thermal expansion greater than 6×10⁻⁷/° C. (e.g., a coefficient of thermal expansion of the sintered glass sheet may be greater than 6×10⁻⁷, 8×10⁻⁷, 10×0⁻⁷, 12×10⁻⁷, 15×10⁻⁷, 30×10⁻⁷/° C. or 40×10⁻⁷/° C.). For example, a glass sheet can have a coefficient of thermal expansion between 6×10⁻⁷/° C. and 30×10⁻⁷/° C. or between 32×10⁻⁷/° C. and 50×10⁻⁷/° C.

Once formed, the glass sheet may be divided into discrete pieces by a suitable cutting device. For example, a laser can be used to cut or trim the glass sheet into smaller pieces. In a further example, a laser can be used to trim the glass sheet from the unsintered soot sheet. Further, before or after cutting, the sintered glass can be subjected to one or more post-sintering processes, such as edge removal, coating, printing, polishing, etc. A long ribbon of sintered glass sheet can be reeled by a reeling device into a roll. Optionally, spacing materials such as paper sheet, cloth, coating materials, etc. can be inserted in between adjacent glass surfaces in the roll to avoid direct contact there between.

The process and apparatus disclosed herein are suited for making soot sheets and sintered glass sheets comprising a low percentage of silica, e.g., “low-silica” glass sheets. By “low-silica” is meant a glass composition comprising less than 50 mole % silica glass, e.g., about than 5, 10, 15, 20, 25, 30, 35, 40, or 45 mole % silica. An example low-silica glass is an aluminoborosilicate glass. Optionally, the second glass soot particles can comprise (e.g., in the form of one or more oxides) titanium, germanium, boron, aluminum, phosphorus, rare earth metals and/or alkaline or alkaline earth metals.

Flexible sintered glass sheets, including long glass ribbons, can be formed. Sintered glass sheets such as low-silica glass sheets can have an average thickness of 200 microns or less (e.g., less than 150, 100, 50, or 25 microns). According to embodiments, however, the sintered glass sheets can have a thickness of 10, 20, 50, 100, 200, 500, 1000, 2000, 5000, 10,000 or 12,500 μm. By controlling width of the deposited soot layer(s), the width of the sintering zone, and the amount of deposition time, it is possible to independently control both the width and the length of sintered glass sheets. A length of the glass sheet can range from about 2.5 cm to 10 km. A width of the glass sheet can range from about 2.5 cm to 2 m.

The process can be used to form high surface quality glass sheets (e.g., glass sheets having low surface waviness, low surface roughness, and which are essentially free from scratches). The above-disclosed process, which can include an initial step of forming a soot sheet on a roll, and a final step of reeling a sintered, flexible glass sheet onto a roll, can be referred to as a “roll-to-roll” process. The resulting glass sheets, including low-silica glass sheets, can be characterized by a number of properties including composition, thickness, surface roughness, surface uniformity, CTE and flatness.

As used herein, “soot layer” or “layer of soot” refers to a stratum of essentially homogeneously-distributed glass particles that are optionally bonded with each other. The layer generally has an average total thickness that is greater than or equal to an average diameter of individual particles. Further, a soot layer may comprise a single soot layer having an essentially homogeneous composition or multiple soot layers each having an essentially homogeneous composition.

In embodiments where the second soot layer comprises multiple layers, one species of glass particles can form one soot layer, while a second species of glass particles can form another soot layer adjacent thereto. Thus, respective soot layers can have distinctive compositional and/or other properties. Moreover, in an interfacial region between the adjacent layers, blending of the two species of particles can occur such that the composition and/or properties at the interface of contiguous layers may deviate from the bulk values associated with each respective layer.

Reference herein to a “glass sheet” includes both sheet materials comprising a plurality of glass soot particles (i.e., soot sheets) and sheet materials made of sintered glass. As is typically understood in the art, a sheet has two major opposing surfaces that are typically substantially parallel to each other, each having an area larger than that of other surfaces. A distance between the two major surfaces at a certain locations is the thickness of the sheet at that particular location. A sheet may have a substantially uniform thickness between the major surfaces, or the thickness can vary spatially either uniformly or non-uniformly. In certain other embodiments, the two major surfaces can be non-parallel, and one or both of the major surfaces can be planar or curved. The glass sheet can be a substantially homogeneous glass sheet or a composite glass sheet having layer-specific attributes.

As used herein, “sintered glass” refers to a glass material having a density of at least 95% of a theoretical density (Dmax) for a glass material having the same chemical composition and microstructure under conditions of standard temperature and pressure (STP) (273 K and 101.325 kPa). In certain embodiments, it is desired that the sintered glass has a density of at least 98%, 99% or 99.9% of Dmax under STP.

Additional aspects of glass sheet formation using a glass soot deposition and sintering process are disclosed in commonly-owned U.S. application Ser. Nos. 11/800,585 and 12/466,939, filed May 7, 2007 and May 15, 2009, respectively, the contents of which are incorporated herein by reference in their entirety.

As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “metal” includes examples having two or more such “metals” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, examples include from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.

It is also noted that recitations herein refer to a component of the present invention being “configured” or “adapted to” function in a particular way. In this respect, such a component is “configured” or “adapted to” embody a particular property, or function in a particular manner, where such recitations are structural recitations as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” or “adapted to” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A method for making a glass sheet, said method comprising: depositing first glass soot particles on a deposition surface to form a supported soot layer; removing the soot layer from the deposition surface to form a first soot sheet, the first soot sheet having a contact surface and a free surface; depositing second glass soot particles on at least one of the contact surface and the free surface to form a composite soot sheet; heating a portion of the composite soot sheet to sinter the second glass soot particles to form a glass sheet that is formed on the first soot sheet; and separating the glass sheet from the first soot sheet.
 2. The method for making a glass sheet of claim 1, wherein the first soot sheet is not sintered during the heating step such that the glass sheet is formed on an unsintered first soot sheet.
 3. The method for making a glass sheet of claim 1, wherein the second glass soot particles are deposited on at least one of the contact surface and the free surface to form a composite soot sheet after the soot layer has been removed from the deposition surface.
 4. The method for making a glass sheet of claim 1, wherein the first glass soot particles have a first sintering temperature and the second glass soot particles have a second sintering temperature, wherein the first sintering temperature is higher than the second sintering temperature.
 5. The method for making a glass sheet of claim 1, wherein the depositing of the second glass soot particles forms a second soot layer having a major surface in contact with at least one of the contact surface and the free surface of the first soot sheet.
 6. The method for making a glass sheet of claim 5, wherein the glass sheet is formed with a major surface in contact with at least one of the contact surface and the free surface of the first soot sheet.
 7. The method for making a glass sheet of claim 1, wherein the first soot sheet is a free-standing, unsupported soot sheet. 